In recent years, development of optical front-end modules for digital coherent receivers has been advancing. As optical front-end modules for receivers, there have been known modules described in Patent Literature 1, for example. A package used for such a module has been standardized in terms of a shape, pin layout and the like. A package is provided with DC leads in two opposed side faces thereof, i.e., 20 DC leads arranged at a pitch of 1.27 mm in each side face. In the package, one of side faces orthogonal to the two opposed side faces is provided with an optical input terminal and the other side face is provided with 20 RF leads arranged at a pitch of 1 mm.
As packages used for aforementioned modules, a metal package is often used in which ceramic circuit boards each having SMT leads are joined by being embedded in side wall faces of the package. For example, as illustrated in FIGS. 1A and 1B, a conventional package is often hermetically sealed in such a way that a lid being a flat plate lid (not illustrated) is seam-welded to a bathtub-shaped package base 10. As illustrated in FIG. 1A, the optical front-end module for receiver includes a waveguide optical element 20, an optical component 30, an optical semiconductor and electronic circuit 40, and the like, which are mounted inside the package 10. Moreover, two opposed side faces of the package 10 are provided with ceramic circuit boards 50 having multiple DC leads 60 connected thereto, and the side surfaces orthogonal to the two opposed side surfaces are provided with a fiber F and output terminals 70.